In general, a lambda probe functions as a device for measuring a gas concentration in an exhaust stream, as part of the design of an internal combustion engine—termed “engine design” below. By means of the measurement result, the so-called lambda ratio of combustion air to fuel, or fuel to exhaust gas, is determined and may be utilized to control the combustion of fuel. A lambda sensor can be implemented as a lambda probe, for example, or—for the purpose of determining the NOx fraction in a gas mixture—can be implemented as a NOx probe. In addition, sensors used to determine the concentration of other gases, for example NH3, CO, or HC can be contemplated, and are termed “lambda probes” in general in the following text.
A correct lambda ratio, or a NOx concentration, or the other concentration values named above which are part of the detection by the lambda probe, are important parameters for the control of the combustion in the combustion chamber of an internal combustion engine, and for enabling adequate exhaust scrubbing by a catalytic converter.
A lambda probe generally has a probe head which is formed as a ceramic measuring element, wherein the same is capable of measuring a gas property, as explained above—generally via a probe voltage. For example, the ceramic measuring element can be realized by means of a concentration cell (Nernst cell), preferably based on an yttrium-doped zirconium dioxide ceramic. The ceramic measuring element can also be realized with a resistance cell (Arrhenius cell), for example with a semi-conducting titanium dioxide ceramic. The measurement result is obtained as the result of ionic conduction by oxygen and/or oxygen occupying a lattice void in the ceramic, according to the measuring principle, with a resulting probe voltage or probe current. Such a probe signal can be a measurement of a difference in concentration between a reference gas and a measured gas (Nernst cell) as the result of a partial pressure difference, or a conductance between a reference gas compartment and a measured gas compartment (Arrhenius cell). For this purpose, the measuring element is regularly heated to temperatures above 650° C., or in some cases—if a YSZ ceramic is used—to temperatures above 300° C.
For example, a discrete-level sensor provides a lambda value which is equal to 1 and a probe voltage between approximately 200 and 800 mV (optimally approx. 450 mV), a lambda value larger than 1 (lean mixture with too much air) at a voltage less than 200 mV, and a lambda value smaller than 1 (rich mixture with too much fuel) at voltages over 800 mV. A discrete-level sensor is characterized in that the characteristic curve is extremely steep in a very narrow intermediate range on either side of lambda=1, meaning between 200 and 800 mV—the so-called lambda window. In this range, the voltage changes, almost in a jump, in dependence on the air-fuel ratio.
In contrast, a wide-band probe is characterized by a suitable layered construction of a pump cell of a measuring gap, and of a Nernst cell, for example. An exhaust gas flow is on one side of the pump cell. An air flow is on the other side of the Nernst cell. The pump cell connects the measuring gap to the exhaust stream via a diffusion channel. The measured gas is constantly held at lambda=1 in the measuring gap via the diffusion channel. The pump current provides information on the lambda ratio in the exhaust gas flow. In the case of a wide-band probe, it is important for the measurement result that the probe is heated.
In general, then, it can be said that lambda probes indirectly measures the current prevailing oxygen concentration in the gaseous medium which surrounds them. The electrical pump current needed for the measuring method—for example, in the case of wide-band probes—is measured and converted via a stored characteristic curve. The precision of this measurement also significantly depends on the calibration interval because—as explained—potentially steep ramps in the probe signal must be taken into consideration as part of the measurement result.
The method of leaving a lambda probe in an engine construction and circulating air around the same for the purpose of determining the probe status is known. For example, the internet document http://de.wikibooks.org/wiki/Mb-Technik/_M07-KE/_Lambda-Sonde explains how it is possible to check the status of a lambda probe as regards to heat generation, the probe voltage signal, or lag smearing of the probe. For this purpose, probe signals or a heating current are measured when the engine is running and at operating temperature.
For the purpose of calibration, the method of circulating pure air around a probe head of the lambda probe is known. In general, boundary conditions of the manufacturer which are suited to such calibration must be observed. A calibration factor can generally be determined by means of analysis software provided by the manufacturer, or ECU software, or suitable evaluation devices—also known as lambdameters—for example. For example, the INCA diagnosis and calibration tool from the Bosch company or the LA4 or AWS2 lambdameter from the ETAS company are known. In general, such calibration or status determination methods are suitable for the LSU or LSU ADV probe from the Bosch company.
A problem with all methods currently known for the calibration of a lambda probe is that the lambda probe remains in the engine construction. It is also a problem that a true calibration of the lambda probe can only take place if it is reasonably correct to assume that the lambda probe is intact; however, if the lambda probe is in a malfunctioning state, a calibration of the same only makes a limited amount of sense.
It would be desirable for the operational test measurement to be able to not only calibrate a lambda probe but also determine the status thereof. It is also desirable to be able to perform an operational test measurement of the lambda probe independently of influences from the engine.